Calibration of the apatite (U-Th)/He thermochronometer on an
exhumed fault block, White Mountains, California
Daniel F. Stockli* Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA
Kenneth A. Farley Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
Trevor A. Dumitru Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA
ABSTRACT
This study provides an empirical calibration of the apatite (U-Th)/He thermochronometer
using the thermal structure derived from an extensive apatite fission-track study of an exhumed,
normal-fault–bounded crustal block in the White Mountains in the western Basin and Range
province. This fault block has been tilted ~25° to the east during extension, exposing a continuous section of rocks previously buried to ~7 km. Apatites yield (U-Th)/He apparent ages of
ca. 50–55 Ma at shallow pre-extensional crustal levels that decrease systematically to ca. 12 Ma
at >4.5 km paleodepth. The ages exhibit a well-defined exhumed apatite He partial retention
zone over a pre-extensional temperature range of ~40–80 °C and are completely reset above
80 °C, as calibrated from the apatite fission-track data. This pattern is in good agreement with
He diffusion behavior predicted by laboratory experiments. The (U-Th)/He and fission-track
methods yield concordant estimates for the timing of the onset of extensional faulting in the
White Mountains ca. 12 Ma. Given the partially overlapping temperature-sensitivity windows,
the (U-Th)/He and fission-track thermochronometers are highly complementary and may be
used together to reconstruct thermal histories over the temperature window of ~40–110 °C.
Keywords: (U-Th)/He dating, fission-track dating, thermochronology, extensional faulting, White
Mountains.
INTRODUCTION
(U-Th)/He dating of the mineral apatite has
attracted considerable interest as a potential lowtemperature thermochronometer (e.g., Zeitler
et al., 1987; Lippolt et al., 1994; Wolf et al.,
1996). Laboratory He-diffusion experiments suggest that this system should be sensitive to crustal
temperatures of ~40–80 °C (Wolf et al., 1996,
1998), lower than any other known thermochronometer. Assuming a mean annual surface
temperature of 10 ± 5 °C and a geothermal gradient of 25 °C/km, this temperature range is equivalent to depths of ~1.2–3 km. Thus the system
can potentially be applied to investigate a variety
of geologic processes in the uppermost part of the
crust, such as mountain building, neotectonics,
and landscape development (e.g., House et al.,
1997; Spotila et al., 1998).
Before the (U-Th)/He system can be applied
with confidence, however, it is essential to demonstrate that the laboratory-derived temperature sensitivity is properly calibrated and that measured
(U-Th)/He ages reliably date cooling events.
Warnock et al. (1997) and House et al. (1999)
compared laboratory-derived He-diffusion data
with He ages obtained on samples collected from
deep wells with reasonably well-known downhole
temperatures. These studies broadly confirm the
expected decrease in apparent age with increasing
downhole temperature, but exhibit considerable
scatter. Possible explanations for this scatter include uncertainties in the thermal history of the
borehole, excess He from U- and Th-bearing mineral inclusions such as zircon, possible inheritance
of He in detrital apatite, and the effects of grainsize variations on the closure temperature.
Another approach for testing the (U-Th)/He
thermochronometer is to apply it to exhumed
rocks, the time-temperature histories of which
have been determined by independent thermochronometers that have fundamentally different
principles and uncertainties. In this paper we apply
(U-Th)/He methods to the White Mountains in
eastern California (Fig. 1), where the thermal
history has been established by extensive apatite
fission-track dating (Stockli, 1999). The northern
White Mountains compose a virtually intact
range-scale crustal block that has been tilted eastward by ~25° during Miocene extension. This tilting has exhumed and exposed rocks that were at
paleodepths of 0 to ~7 km before extension began
(Stockli, 1999). This near-ideal setting permits
(U-Th)/He and fission-track samples to be analyzed from a range of pre-extensional paleodepths
(and thus paleotemperatures), to characterize the
in situ behavior of the (U-Th)/He system.
*Present address: Division of Geological and
Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. E-mail:
[email protected].
WHITE MOUNTAINS FAULT BLOCK
The Basin and Range province is a broad area
of Tertiary extension in the western United States.
Many areas of the province exhibit a distinctive
structural style in which the individual ranges consist of more or less intact, exhumed, crustal-scale
footwall blocks that are bounded along one flank
by major high-angle normal faults and that have
been tilted during major fault offset (e.g., Stewart,
1980). The White Mountains are an excellent
example of such a block, having undergone substantial eastward tilting that was accommodated
by normal displacement along the White Mountains fault zone (Fig. 1). Detailed mapping and
structural analyses suggest that the interior of the
White Mountains is essentially undeformed and
rigid. A sequence of pre-extensional Cenozoic volcanic rocks (ca. 12–15 Ma) unconformably overlies basement rocks on the eastern flank of the
range and records ~25° of eastward tilting since
the middle Miocene (Fig. 1) (Stockli, 1999).
THERMOCHRONOLOGY ON
EXHUMED FOOTWALL ROCKS
Pre-exhumation fission-track and (U-Th)/He
ages are expected to vary systematically with
depth and thus burial temperature in the stable
crust (Green et al., 1989; Wolf et al., 1996, 1998;
Dumitru, 2000). The increase in depth and temperature results in a measurable reduction of
apparent ages by thermally induced annealing of
fission tracks and diffusive loss of He. Figure 2
shows a schematic structural model of the exhumation and cooling of extensional crustal fault
blocks. If fault slip has been rapid and of sufficient
magnitude to exhume samples from the zeroretention zones, (U-Th)/He and fission-track ages
will directly date the timing of faulting and footwall exhumation (e.g., Fitzgerald et al., 1991;
Miller et al., 1999). At increasingly shallow paleodepths, apparent ages will become older, because
the isotopic clocks at least partially accumulate
before exhumation commences. The observed
(U-Th)/He ages and fission-track data in these
exhumed partial-retention zones may be used to
estimate the pre-extension paleotemperatures of
samples from various depths.
APATITE (U-Th)/He AND FISSION-TRACK
THERMOCHRONOLOGY
Fission-track dating of apatite is based on the
decay of trace 238U by spontaneous nuclear fission (e.g., Fleischer et al., 1975; Dumitru, 2000).
The use of apatite fission-track methods for
thermochronologic analysis depends on the fact
that tracks are partially or entirely annealed
Data Repository item 2000102 contains additional material related to this article.
Geology; November 2000; v. 28; no. 11; p. 983–986; 3 figures.
983
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Figure 1. Shaded relief map and cross section (A–A’) of northern White Mountains, east-central
California and west-central Nevada, illustrating geometry of east-tilted White Mountains footwall
block. Tilt block is structurally controlled by west-dipping White Mountains fault zone (WMFZ),
which accommodates ~8–9 km of normal displacement, resulting in ~25° of eastward tilt as
recorded by Tertiary volcanic sequences along eastern flank of range.
10 °C
0 km
He PRZ
2.5 km
BEFORE
EXTENSION
FT PAZ
50 °C
100 °C
5 km
150 °C
7.5 km
THERMOCHRONOLOGY
SAMPLING TARGET AREA
footwall
erosion
AFTER
EXTENSION
hanging-wall
basin fill
Figure 2. Simplified model illustrating exhumation of footwall rocks during extension. Apatite
fission-track (FT) and (U-Th)/He sampling transects are devised to obtain samples from largest
possible range of pre-extensional paleodepths exposed in intact, tilted fault blocks. PRZ—partial
retention zone.
984
(erased) by thermally induced recrystallization at
elevated temperatures, causing reductions in both
the lengths of individual tracks and the fissiontrack ages. Fission tracks are shortened and
partially erased by annealing in about the temperature range of 60–110 °C, termed the partialannealing zone (PAZ); essentially no annealing
occurs at lower temperatures, and total erasure
occurs at higher temperatures (e.g., Laslett et al.,
1987; Green et al., 1989).
(U-Th)/He dating is based on the α-decay of
235U, 238U, and 232Th series nuclides. Extrapolation of laboratory volume-diffusion kinetic
parameters to geologic time scales indicates that
He is completely expelled from apatite above
~80 °C and almost totally retained below ~40 °C
(Wolf et al., 1996, 1998). Diffusion experiments
further suggest that the apatite (U-Th)/He system
has a closure temperature of ~65–75°C, assuming a constant cooling rate of 10 °C/m.y. (Farley,
2000). He diffusivity correlates with the physical
dimensions of Durango apatite, indicating that
the diffusion domain is the grain itself, so grain
size has a small effect on the closure temperature
(Farley, 2000).
APATITE SAMPLES AND METHODS
We collected 15 samples of Cretaceous granitic basement along an east-west transect
across the northern White Mountains; these
rocks effectively sample the entire exposed
structural section of the crustal block (Fig. 1).
Experience has shown that the biggest hurdle to
accurate and reproducible He age determinations is isolation of apatite grains that are free of
U-, Th-, and He-bearing impurities, in particular zircon microlites and fluid inclusions (e.g.,
Warnock et al., 1997; House et al., 1997). In
order to eliminate suspect grains, every apatite
was inspected prior to analysis under a binocular microscope at 125× magnification. Unrecognized zircon inclusions could be detected by a
re-extraction process incorporated within the
standard He furnace degassing procedure
(House et al., 1999). All analyzed apatite grains
in this study were euhedral with a nearly invariant grain size of ~170 ± 20 µm in length and
~70 ± 10 µm in radius. The selection of a uniform grain size should minimize differences in
diffusion behavior related to grain size (Farley,
2000). These measurements were also used to
correct for α emission (Farley et al., 1996).
The 10 crystals analyzed from each sample
show that all apatites are monocompositional
fluorapatite with Cl contents of <0.02 wt%
(Table 11). Qualitative optical inspection of induced fission-track distributions on external
1GSA Data Repository item 2000102, Thermochronological and compositional data and methodology, is available on request from the Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140,
[email protected], or at www.geosociety.org/
pubs/ft2000.htm.
GEOLOGY, November 2000
detectors from irradiated fission-track samples
suggests that U zoning in all apatite grains is minimal. Thus α-emission corrections, which assume
homogeneous U and Th distributions, will be
accurate in these rocks (Farley et al., 1996).
APATITE (U-Th)/He AND
FISSION-TRACK RESULTS
The fission-track age and mean track length
data outline a well-defined exhumed PAZ (Fig. 3;
Table 2 [see footnote 1]). The observed patterns
correspond well with expected apparent age
versus depth patterns (e.g., Gleadow et al., 1986;
Green et al., 1989) and with predicted apparent
age versus paleodepth patterns (e.g., Miller et al.,
1999). Fission-track ages decrease systematically with increasing structural depth from 59.3
± 3.8 Ma at ~1.7 km to 12.1 ± 1.5 Ma at ~6.2 km
(Fig. 3). Below the base of the PAZ at ~6.2 km
depth (T >~110 °C), fission-track ages are invariant ca. 12 Ma, indicating the timing of rapid
exhumation of the White Mountains. The vertical separation between the base of the PAZ
(T ~110 °C) and the Miocene basal unconformity yields a paleogeothermal gradient of
15 ± 2 °C, assuming a mean annual surface temperature of ~10 ± 5 °C. This reconstruction of
the pre-extensional thermal state of the crust provides a thermal reference frame for evaluating
the (U-Th)/He thermochronometer.
Apparent (U-Th)/He ages similarly decrease
with increasing paleodepth, from 57.5 ± 0.9 Ma
to 12.1 ± 1.1 Ma, defining a well-developed
partial retention zone (PRZ) (Fig. 3; Table 3
[see footnote 1]). A marked inflection point in the
age versus depth profile marks the base of the
PRZ at ~4.5 km. The apparent age behavior is
consistent with theoretically predicted apparent
age versus depth or temperature profiles (e.g.,
Wolf et al., 1998) and borehole studies (Warnock
et al., 1997; House et al., 1999). Below the PRZ,
(U-Th)/He ages are essentially invariant, clustering ca. 12 Ma. At the deepest structural levels
exposed in this area, the (U-Th)/He results trend
toward younger ages as a reflection of renewed
Pliocene faulting (Stockli, 1999). Apparent
(U-Th)/He ages are consistently equal to or
younger than apatite fission-track ages from the
same sample, with two exceptions.
1. Sample 96BR217 (Table 3; see footnote 1)
from the western range front exhibits anomalously old (U-Th)/He ages. Three analyses
yielded ages ranging from 45 to 93 Ma, substantially older than the corresponding fission-track
age of 26 Ma. Re-extraction data do not point to
U- or Th-bearing inclusions as the probable
explanation for these anomalously old ages.
Optical examination of grains at 1250× revealed
the presence of abundant elongate, minute fluid
inclusions. The presence of 4He in fluid inclu-
0
Distance below Tertiary unconformity (km)
Miocene paleo-geothermal
gradient = 15 ± 2 °C/km
He
partially reset by
Pliocene volcanism
sions might represent a feasible explanation, but
in-vacuum crushing experiments would be required to test this possibility.
2. One granitic basement sample was collected
~4 m below the Tertiary basal unconformity,
which is overlain by ~200 m of Miocene andesite
dated as 5.70 ± 0.04 Ma (Stockli, 1999). The apatite fission-track age of 6.9 ± 1.2 Ma is concordant
with the 40Ar/ 39Ar age of the andesite. However,
two (U-Th)/He analyses yielded apparent ages of
11.2 ± 0.8 and 11.6 ± 1.4 Ma, distinctly older than
the 40Ar/ 39Ar age of the overlying andesite.
Whereas the fission-track system apparently dates
the heating and subsequent cooling of the basement during andesite emplacement, He loss was
only partial. This discrepancy arises from the fact
that the rate of diffusive loss of He depends on
both the diffusivity and the concentration gradient. As diffusion proceeds and the concentration
gradient becomes shallower, the rate of loss drops
dramatically. For example, on the basis of diffusion measurements (Wolf et al., 1996), 45% of the
He in a Durango apatite (180 µm diameter) will
be lost in 3.3 h at 360 °C; losing the next 45%
requires almost an order of magnitude more time.
In contrast, essentially all fission tracks disappear
in Durango apatite in just 1 hr at the same temperature. The effect of short-term heating on apatite
(U-Th)/He and fission-track ages warrants further
investigation, but laboratory observations and this
FT
FT
2 ~40 °C
m
50 µ
exhumed
PRZ
m
80 µ
4
~80 °C
~60 °C
exhumed
FT PAZ
6
~110 °C
rapid exhumation at ca. 12 Ma
Pliocene faulting
8
0
20
40
Apparent age (Ma)
60
80
10
12
14
Mean track length (µm)
Figure 3. Summary diagram of integrated apatite (U-Th)/He and fission-track (FT) data from northern White Mountains. Apatite fission-track data
establish thermal reference system that allows for empirical calibration of (U-Th)/He thermochronometer. Observed partial retention zone (PRZ)
is in excellent agreement with forward-modeled PRZ for range of grain sizes (50–80 µm radii, dashed curves), assuming isothermal holding for
43 m.y. prior to rapid exhumation at 12 Ma. Samples from below exhumed PRZ are invariant in age and directly date time of inception of footwall
cooling and extensional faulting.
GEOLOGY, November 2000
985
White Mountains sample suggest that fissiontrack ages younger than He ages will occur when
the duration of heating is short compared to radiogenic ingrowth.
DISCUSSION AND CONCLUSIONS
This study confirms that He diffusion parameters derived from laboratory experiments can be
extrapolated to geologic time scales. The reconstruction of the pre-extensional thermal state of the
crust using the apatite fission-track system confirms that the (U-Th)/He thermochronometer is
characterized by a partial retention zone between
~40 and 80 °C (Fig. 3). This result is in good
agreement with the ~40–80 °C range derived
from forward modeling results of the He PRZ
(Fig. 3) and represents robust geologic evidence
that the temperatures calculated for the (U-Th)/He
partial retention zone from laboratory data are
correct. This study also intercalibrates apatite
fission-track and (U-Th)/He thermochronology
and shows that the two low-temperature thermochronometers yield consistent estimates for the
time of onset of rapid footwall exhumation of the
White Mountains fault block as well as for the
pre-extensional paleogeothermal gradient. Compared to the first geologic calibration studies using
borehole data (Warnock et al., 1997; House et al.,
1999), these data yield well-behaved apparent-age
versus depth profiles with relatively little scatter
(Fig. 3). This behavior can be attributed in part to
recent advances in analytical procedures such as
the detection of zircon inclusions as well to the
nature of the apatite samples. In this study, only
plutonic, inclusion-free, monocompositional, and
unzoned (at least in U) apatite grains with a uniform grain size were used to minimize differences
in diffusion behavior.
In interpreting (U-Th)/He apparent ages, it
must be recognized that they represent total gas
ages, much like K/Ar ages. During nonmonotonic cooling or isothermal periods, partial diffusive loss and retention of 4He substantially reduces the age of a sample that resided at
temperatures between ~40 and 80 °C. Such apparent (U-Th)/He ages cannot generally be interpreted as directly dating when a sample cooled
through a specific closure temperature (Dodson,
1973). In this case study, only 5 of 15 (U-Th)/He
samples directly date the timing of cooling and
footwall exhumation at 12 Ma, whereas most
samples yield apparent ages that do not directly
date a specific cooling event. At this point,
proper interpretation of a given (U-Th)/He age
from a single sample is possible only with corroborating evidence, such as apatite fission-track
data from the same sample and/or (U-Th)/He
versus paleodepth sample arrays.
This study demonstrates that the apatite
(U-Th)/He thermochronometer provides a means
to reliably quantify thermal histories between 40
and 80 °C. A wide variety of important geologic
processes in the upper ~1–5 km of the crust can
986
potentially be investigated by (U-Th)/He dating
methods and by integrated apatite (U-Th)/He and
fission-track studies. Given their partially overlapping sensitivity windows of ~40–80 °C and
60–110 °C, the apatite (U-Th)/He and fissiontrack thermochronometers are complementary
and can be used together to robustly define lowtemperature thermal histories.
ACKNOWLEDGMENTS
This project was supported by National Science
Foundation grants EAR-9417937, EAR-9725371 (to
E. Miller), and EAR-9805226 (to Farley); a Stanford
University McGee grant and a University of California
White Mountains Research Station fellowship to
Stockli; and a Packard Foundation fellowship to
Farley. We thank L. Gilley, L. Hedges, J. Hourigan,
and B. Surpless for assistance in the field and laboratory, B. Jones for assistance with electron microprobe
work, and the Oregon State University reactor facility
for sample irradiation.
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Manuscript received March 23, 2000
Revised manuscript received July 24, 2000
Manuscript accepted July 31, 2000
GEOLOGY, November 2000